An electrolysis process is a basic technology in which so far as an aqueous solution system is concerned, hydrogen, oxygen, ozone, hydrogen peroxide and the like can be generated by controlling a chemical reaction on the electrode surface by utilizing clean electric energy and which is used for various purposes including electrolysis of sodium chloride, electrolytic plating, metal electrowinning and the like as industrial electrolysis. In recent years, the electrolysis process is being utilized as a wastewater treatment because it is possible to indirectly decompose organic pollutants, or to adsorb the pollutants onto an electrode, thereby directly electrolyzing them.
In an oxidation reaction on an anode in the electrolysis, it is known that an oxidizing agent (effective chlorine, ozone, etc.) which is effective for the water treatment is formed; and that an active species such as an OH radical is also partially generated, and water containing the same is used for various purposes as a name such as active water, functional water, ion water or sterilized water. Such an electrolysis process is put to practical use. However, it has been pointed out that the objective reaction does not sufficiently proceed depending upon electrode materials. In general, according to an anodic oxidation reaction in electrolysis in an aqueous solution, electrolysis products resulting from water as a raw material are formed. However, in many cases, oxidation of other coexistent substances does not easily proceed in an electrode catalyst having high reactivity against discharge of water.
Examples of electrode (anode) catalyst materials for electrolysis for performing the oxidation include lead oxide, tin oxide, platinum group metals and oxides thereof, carbon and the like. Materials which can be used as an electrode substrate are required to have corrosion resistance from the viewpoint of a long life and such that contamination on the treated surface does not occur; and the anode substrate is limited to valve metals such as titanium, and alloys thereof, and the electrode catalyst is limited to noble metals such as platinum and iridium, and oxides thereof. However, it is known that even if such an expensive material is used, when a current is allowed to flow, the material is wasted according to the current density or time and flows out into the solution, so that electrodes having more excellent corrosion resistance are desired. Graphite or an amorphous carbon material has hitherto been used as an electrode material. However, such a material has exhaustion properties, and in particular, is conspicuously exhausted under anodic polarization.
On the other hand, diamond is excellent in thermal conductivity, optical permeability and durability against high temperatures and oxidation. In particular, since it is possible to control electric conductivity by doping, diamond has been regarded as promising as a semiconductor device or an energy conversion device. However, Patent Document 1 discloses an application of a diamond electrode, as a sensor, to which conductivity is imparted by ion injection.
Swain, et al. (Non-Patent Document 1) reported stability of diamond as an electrode for electrochemical reaction in an acidic electrolytic solution and suggested that the diamond is far excellent as compared with other carbon materials. Detailed descriptions regarding fundamental electrochemical characteristics are given in Non-Patent Document 2.
Patent Document 2 suggests that organic wastewater can be decomposed by using diamond as an anode material. Patent Document 3 proposes a method of electrochemically treating an organic material by using conductive diamond as an anode and cathode. Also, Patent Document 4 proposes a method of performing a water treatment by using a conductive diamond electrode as an anode and a gas diffusion cathode for generating hydrogen peroxide as a cathode.
Any industrial application in a high potential region in the case of a large current density has not been reported yet. However, Patent Document 5 reports that the diamond electrode is inert against a decomposition reaction of water; and that ozone is generated in addition to oxygen in an oxidation reaction. Patent Document 6 discloses that the diamond electrode can be utilized in electrolysis of a molten salt.
In the electrolysis process using diamond as an electrode for electrolysis, the reaction efficiency is enhanced as compared with the case using a conventional electrode. However, there was the case where the life is poor, so that the diamond electrode is unable to correspond thereto depending upon the application field.
As causes thereof, it may be estimated that since active sites of the diamond surface are small in the existing density as compared with other electrode materials and smooth in the geometrical shape (the electrochemical double-layer capacity of the diamond electrode is only about 1/100 as compared with the electrochemical double-layer capacity of a platinum electrode), an actual current density increases as compared with a given current density, so that electrode depletion by electrolysis is easily caused.
In recent years, there has been devised a process of etching the diamond surface, on which a porous masking material is placed with oxygen plasma, thereby producing a diamond electrode having a honeycomb shape having pores of several ten nm and pitches of about 100 nm (depth: several μm) depending on the mask specifications (Non-Patent Document 3); and there has also been reported a technology in which other catalyst such as platinum is formed thereon (Non-Patent Document 4).
However, according to this method, it was difficult to apply them to industrial electrodes, and also, there was a limit in making the porosity finer. Under such circumstances, it has become necessary to further develop a porous diamond electrode which can be utilized for the industrial electrolysis.
Patent Document 7 discloses a production process of a diamond electrode having high electrode activity and a large number of micropores as compared with the conventional diamond electrodes. By thermally treating metal particles deposited on the diamond layer coated on the substrate surface in a reducing gas atmosphere, a carbon reduction reaction using the foregoing metal as a catalyst is allowed to proceed, thereby forming micropores on the surface of the diamond layer. Since the metal particles supported on the diamond layer surface are utilized, a diamond layer on which micropores are formed on an atomic level or a level close thereto, or diamond particles are obtained. The foregoing micropores are intended to increase the surface area of the diamond layer as a catalyst, thereby enhancing the catalytic activity. However, according to this process, it is necessary to deposit the metal particles on the diamond layer surface by a vapor deposition method or the like and also to perform the thermal treatment in a reducing gas atmosphere.
On the other hand, in high-speed zing plating, electrolytic copper foil production and the like which are represented by EGL (Electro Galvanizing Line), a so-called dimensionally stable electrode (DSE) obtained by coating a platinum group metal or an oxide thereon on the surface of a valve metal such as titanium, is used.
DSE is used for various purposes because it shows remarkably excellent stability as compared with other electrodes, is free from the contamination of impurities into the formed metal and has a high quality. It is known that this dimensionally stable electrode is used for electrolysis of sodium chloride and shows excellent stability. However, when the dimensionally stable electrode is used in a plating bath or the like, oxidative corrosion of titanium as a substrate proceeds, and nevertheless the coating layer of a platinum group metal oxide remains, the electrolysis cannot be performed. Therefore, countermeasures to these problems are important. For example, Patent Document 8 and Patent Document 9 disclose technologies in which an interlayer of a metal such as tantalum, or an alloy thereof or the like is formed between the substrate and the electrode active material, thereby improving durability of the electrode.
Patent Document 10 discloses a technology in which a diamond layer is formed as an interlayer on a substrate, and thereafter, a platinum group metal catalyst is formed. However, this diamond interlayer has a smoothly deposited structure, and in the case of forming the catalyst in this diamond interlayer, in particular, in the case of coating a precursor solution in which a catalyst raw material is dissolved on the substrate and forming a catalyst layer by a thermal treatment, since an adequate anchor is not formed, coating of the catalyst is difficult, and falling-off of the formed catalyst is easily caused. Thus, this technology was not put into practical use.
Patent Document 11 discloses that a large number of conductive diamond particles in which a carbon reactive catalytic metal is supported on micropores formed on the surface of the diamond particle is mixed with a binder to form an electrode catalyst for electrochemical reaction. This catalyst can be used as an oxygen electrode or a hydrogen electrode of a fuel cell. The metal catalyst supported within the micropore is higher in the catalyst stability (aggregation inhibition or falling-off prevention) than metal catalysts supported on the particle surface and high activity and long life can be achieved thereby with a small amount of the metal, so that the use amount of the expensive metal can be reduced. Though this patent document discloses a technology for forming the catalyst on the diamond particle, it does not describe an application to a large-sized electrode for industrial electrolysis.